A multi-stage active vibration isolation system

A major obstacle to the low-frequency operation of a gravitational wave interferometer is seismic noise. The current plans for the LIGO (Laser Interferometer Gravitational wave Observatory) receiver now under construction project that ground noise will limit the operating band to frequencies above 40 Hz.; We describe the JILA active vibration isolation system. This device is being constructed to demonstrate the technology needed for useful reduction of low frequency seismic noise in a gravitational wave interferometer. It consists of three spring-mounted stages, each of which provides both active and passive isolation. All of the control loops on the first two of the three stages have been closed. Together they can reduce large vibrations by at least 70 dB in both vertical and horizontal directions at 1.5 Hz and above. Interferometric readouts have also been incorporated in the second stage seismometers as servo error signals, which should allow for reduction of displacement noise on that stage to the 10{dollar}sp{lcub}-13{rcub}{dollar}m/{dollar}rmsqrt{lcub}Hz{rcub}{dollar} level above 1.5 Hz.